1. Field of the Invention
This invention relates to a hydrophilic molding compound of homopolyester or copolyester, a process for the preparation of the same and to the utilization of this molding compound as material for the formation of hydrophilic homopolyester or copolyester fibers.
2. Description of the Prior Art
It is known that the wearing hygiene and, consequently, also the wearing comfort, of a textile depends essentially upon its heat and moisture transport ability. Natural fibers are hydrophilic; i.e., they take up substantial moisture from the air and have a high absorbency and a large water retention capacity. However, when the body releases heat and starts to perspire, natural fibers have drawbacks as compared to polyester. For example, in swelling, the fiber cross section of wool increases some 25%, that of polyester only by 1% (Robinson, Textilpraxis International, 1976, p. 1180). As a result, and particularly with dense textiles, the permeability to air and, hence, the direct moisture transport through the textile, are inhibited. Moreover, during the absorption of water by wool, much more additional heat is generated, namely 113 J/g water (27 cal/g water), than during the absorption of water by polyester, namely 3.35 J/g water (0.8 cal/g water) (Robinson Textilveredlung, 1977, p. 264). Closely related to this thermal effect is that with increasing temperature, the absorbency of wool declines significantly. Finally, textiles of polyester dry much faster than those of wool. The advantages of polyester in this context are essentially based upon the fact that it takes up much less water. On the other hand, the low water uptake is a cause of the unsatisfactory wearing hygiene and of the low wearing comfort of polyester.
It has long been desired to eliminate this drawback of polyester; that is, to improve its moisture uptake and water retention while not compromising its other favorable characteristics.
Among other things, chemical modification of either the entire fiber or of merely the fiber surface has been carried out in an attempt to improve polyester's hydrophilic nature. This course has, however, not resulted in any convincing success.
Actually, it is considered to be more advantageous to physically change the fiber structure, e.g., by increasing the absorbent fiber surface. Melliand Textilberichte 1/1977, pp. 11-12, describes a proposed structure for such fiber, which, in the case of polyacrylonitrile fibers, has apparently already been realized. This polyacrylonitrile fiber consists of a core comprising a large number of capillary pores and a skin with a great many fine channels capable of conveying water from the outside into the porous inside of the fiber. The skin protects the internal pore system and ensures smooth processing.
According to a process described in German Patent Application OS No. 25 54 124, such hydrophilic polyacrylonitrile fibers are obtained by a dry-spinning process whereby one adds to the spinning solvent which is selected from the group comprising dimethyl acetamide, dimethyl sulfoxide, N-methyl pyrrolidone or dimethyl formamide, from 5 to 50 wt. %, based on solvent and solid, of a liquid having a higher boiling point than the spinning solvent used, which mixes well with the spinning solvent and with water and which is a non-solvent of the polymer to be spun. Examples of these liquids are: alkyl ethers and alkyl esters of multivalent alcohols, high boiling alcohols, esters and ketones. The use of glycerin is preferred. The filaments obtained in this manner have a skin/core structure, a porous core with pores of an average diameter of 0.5 to 1 microns (=5000 to 10000 .ANG.), a moisture uptake of about 2 to 5% (at 65% relative humidity and 21.degree. C.) and a water retention of 10 to 30%.
Dry-spun fibers of skin/core structure obtained by acrylonitrile polymerizates having a moisture uptake of at least 7% (at 65% RH and 21.degree. C.) and a water retention of at least 25% are described in German Patent Application OS No. 26 07 071. These fibers are obtained by spinning a COOH group containing acrylonitrile polymer from a solvent to which is added from 5 to 50 wt. %, based on the total weight of the solution, of a compound having a boiling point higher than that of the spinning solvent, which is miscible with water and with the spinning solvent and does not dissolve the copolymer, followed by washing out the compound added to the solvent from the freshly spun thread and by converting all or part of the carboxyl groups into the salt.
While the above described process is applicable to polyacrylonitrile, it cannot be used on a production scale with most polyesters to improve their hydrophilic properties. Polyacrylonitrile is obtained by a dry-spinning process, that is, it is spun from an organic solvent at comparatively low temperatures. Conversely, polyesters are spun on a production scale at nearly 300.degree. C. from a melt, i.e., under much more critical conditions. Moreover, nonmodified polyacrylonitrile has already a comparatively high moisture uptake of about 1.5%. Unmodified polyester has, by contrast, a much lower moisture uptake of merely 0.3 to 0.6%.
There are also a great many porous polyester fibers which may be obtained, as by special drawing methods or by blowing with inert gases. Like the above-described polyacrylonitrile fibers, they exhibit large pores visible under the light microscope, but which do not significantly increase the moisture uptake.
German Patent Application OS No. 27 55 341 describes hydrophilic polyester fibers characterized in that they possess a stable microporous system subject to capillary condensation at 20.degree. C. and a relative humidity of less than 97%, and which have at least 40.degree. C. and 92% relative humidity, a moisture uptake of more than 2 wt. %, whereby the moisture uptake attributable to capillary condensation represents at least 25%. Said fibers are obtained by spinning a polymer compound containing from 1 to 20 wt. % of one or more oxalato complexes of the general formula Me.sub.n [Z(C.sub.2 O.sub.4).sub.m ] (Me=alkali metal, Z=central atom such as e.g., Al, Fe, Zn, Zr, Cr, Mg, Cu, n=1, 2, 3 or 4, m=2, 3 or 4), by drawing the resulting thread and by hydrosetting at temperatures ranging from 90.degree. to 170.degree. C. in the presence of liquid water.
German Patent Application OS No. 23 64 628 describes hydrophilic structures of water-insoluble polymers. Mention is made of, among others, polyester and polyethylene terephthalate which contains particles of otherwise water-soluble cellulose ethers which, by cross linking or modification, have become almost completely or completely insoluble, but still able to take up water. These particles are either evenly distributed in the polymer compound or cover the surface thereof. The particle size of the particles in the polymer is 10 micron to 2 mm. It will be recognized, however, that particles of such large size exhibit serious drawbacks, especially in terms of the mechanical properties of the structures obtained from these mixtures.
A new route to obtain hydrophilic polyesters has been found. The subject matter of the invention is a hydrophilic molding compound of homopolyester and/or copolyester, characterized in that it contains 1 to 20 wt. % of an alkali and/or alkaline earth salt of salt-forming polymers and/or copolymers of ethylene-unsaturated monomers as filler in a finely dispersed distribution.
The term polyester refers to both homopolyester and copolyester of terephthalic and isophthalic acid with one or more of the following polyvalent alcohols: ethylene glycol; diethylene glycol; 1,2-propanediol; 1,3-propanediol and 1,4-butanediol. The polyester is preferably composed of at least 80 wt. %, based on the polyester component, of polyethylene terephthalate.
A finely dispersed distribution in the sense of the application refers to dispersions of the fillers in the polyester whose maximal particle size does not substantially exceed 2 microns. The maximum particle size is preferably 1 micron. Depending on end use, the filler component amounts to preferably 3 to 12 weight percent. When used as filament or fiber material, the filler constituent in the special development of the invention amounts to 5-8 weight percent.
The salt-forming polymers and/or copolymers are formed from one or more monomers selected from the group comprising vinyl sulfonic acid, acrylamidoalkylene sulfonic acid, sulfonated styrene, acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, crotonic acid, vinyl phosphonic acid, monoalkyl ester of maleic acid and monoalkylester of fumaric acid.
In many cases, it has been found advantageous for the salt-forming polymers to also include comonomers selected from the group comprising styrene, alpha-olefins, vinyl alkyl ethers and esters of the aforementioned monomers.
In the molding compounds of the invention, the additives rendering the polyester hydrophilic are present as calcium or magnesium salt or as calcium or magnesium double salt with lithium, potassium or sodium. Pure lithium, potassium or sodium salts of the salt-forming polymers, while imparting to the polyester compound a high moisture absorbency, often produce an undesirable increase in melt viscosity of the molding compound, thus complicating polycondensation and processing. Although the pure calcium and magnesium salts of the invention result in a lower moisture absorbency of the polyester compound than that obtained with pure alkali salts, they do not substantially raise the melt viscosity. The double salts of a calcium or a magnesium salt with a lithium, potassium or sodium salt display very good characteristics. These double salts of the invention combine the properties of both alkali salts and alkaline earth salts, i.e., they impart a high moisture absorbency without significant increase in the melt viscosity of the polyester. The alkali salt and/or alkaline earth salt of the salt-forming polymer is preferably a double salt of calcium or magnesium with 20 to 70 equivalent percent, especially 40 to 60 equivalent percent of lithium, potassium or sodium salt.
The alkali salt and/or alkaline earth salts of the salt-forming polymers can be used with and without cross linking. It is surprising to find that even after repeated washing, non-cross linked polymer salts are not or only slightly removed. Cross linking agents for this purpose are the conventional agents such as divinyl benzene or divinyl ethylene glycol with divinyl benzene being preferred. The degree of desirable cross linking is determined by the type of alkali salt or alkaline earth salt of the salt-forming polymers, whereby the cross linking agent component may amount up to 10 mole % based on the alkali salt or alkaline earth salt of the salt-forming polymer, preferably 2 to 5 mole percent.
The molding compounds of the invention of the invention may, of course, contain additions of conventional polyester stabilizers, pigments, nucleating agents, viscosity-controlling agents, etc. The molding compounds of the invention can be processed to shaped articles whose physical properties correspond to those of polyester materials with comparable quantities of additives. This applies specifically in cases where the molding compounds of the invention are used for the production of fibers. In the case of non-textile end uses, it is possible to increase the polymer salt content of the polyester to up to 20 wt. %. By doing so, the hydrophilic nature of the end product is considerably enhanced.
The alkali salts or alkaline earth salts of salt-forming polymers used within the scope of the invention may be obtained by any known polymerization process. However, use is preferably made of salts of polymers obtained from polymerizations wherein the monomers or, as the case may be, the comonomers of the polymer salts are polymerized or copolymerized in an inert solvent in the presence of a wetting agent and an initiator at temperatures between 80.degree. and 100.degree. C., followed by the conversion of the resulting homopolymer and copolymer to the salt(s) by the addition of alkali hydroxide and/or alkaline earth hydroxide with the application of heat for several hours, followed by separation, washing and drying of said salt(s).
In the preferred version of the process, inert solvents are expediently selected which do not cause swelling of the polymer salts and yield them in a form allowing favorable filtration. Examples of suitable solvents are: benzene, toluene, xylene, hydrocarbons (ligroin) and chlorinated aromatics such as chlorobenzene. Suitable wetting agents are: non-ionic surfactants such as, e.g., ethoxylated fatty alcohols or ethoxylated nonyl phenol.
As initiators, use is made of conventional compounds, e.g., peroxides like dibenzoylperoxide, tert.butylperoxy-2-ethyl-hexanoate, tert.butylperoxy-isopropyl carbonate, or azodiisobutyronitrile. Initiators are used in conventional quantities, e.g., in amounts of 0.2 to 0.4 wt. %, based on monomers. The monomer(s) is/are preferably added together with the cross linking agent portion by portion during the polymerization. This ensures uniform cross linking and a substantially uniform particle size.
Conversion of homopolymers or copolymers into the polymer salts can be accomplished without separation of the solid in the polymerization system by addition of the alkaline earth hydroxide or, as the case may be, the alkali hydroxide followed by heating for several hours. The heating temperature is preferably the boiling temperature of the solvent. As a rule, 24 to 48 hours are required for a practically quantitative salt formation. The salts are then separated and washed in a suitable liquid. Suitable washing liquids are methanol and ethanol.
The polymer salts can also be obtained by first heating the polymers in one of the above-cited solvents with solid calcium or magnesium hydroxide, followed by addition of solid lithium hydroxide, potassium hydroxide or sodium hydroxide, separation of the solids by suction, by take-up in alcohol and continued heating until total neutralization. This procedure has the advantage that neutralization, which proceeds slowly in the cited solvents, is much more rapid in the alcohol medium. It is also possible in the preparation of the polymer salts to start with the salt form of the monomers and to polymerize the salts. Polymerization can be carried out in an alcohol medium, e.g., in methanol with 10 to 15 wt. % water. With this method, the cross linking agent can also be added together with the monomer, portion by portion, to the polymerization system. Azoisobutyronitrile is a suitable initiator. The mole ratio of initiator to monomer can be 1:500, for which the polymerization time is about 24 hours. The advantage of this procedure is that salt formation with the monomer is substantially faster and more economical, and that, moreover, salt formation and polymerization are carried out in one reactor using only one solvent. The polymer salt is obtained in a finely dispersed form, is separated by suction and the monomer is washed out with, e.g., methanol.
Polymerization or copolymerization of vinylsulfonic acid and/or acrylamidoalkylene sulfonic acid is expediently carried out in water, preferably at 50.degree. to 60.degree. C., in the presence of a redox (oxidation-reduction) catalyst. After addition of alkali hydroxide or alkaline earth hydroxide, heating is continued for several hours, the alkali salt or alkaline earth salt is precipitated by addition of, e.g., alcohol, and then, after separation of the salt, the latter is dried.
The subject matter of the invention is also a process for the preparation of the molding compounds of the invention by incorporating into the polyester compound of from 1-20 wt. % of an alkali salt and/or alkaline earth salt of a salt-forming polymer and/or copolymer. Preferably, the proportion of alkali salt and/or alkaline earth salt of a salt-forming polymer and/or copolymer to be incorporated represents 3 to 12 wt. % and, in a particularly useful embodiment, from 5 to 8 wt. %.
The dried polymer salts are suspended, preferably in the polyvalent alcohols participating in the polyester synthesis. The solid content of the suspension is preferably 20 to 25 wt. %. To increase the dispersion of the polymer salts, the suspension of the salt(s) is preferably homogenized in polyvalent alcohol before addition to the polyester starting materials. This can be accomplished, for example, in a Pearl mill. The salt is ground so that the maximum particle size is preferably less than 1 micron. Polycondensation is then carried out in the conventional manner in conjunction with standard transesterification and polycondensation catalysts. The hydrophilic molding compounds of the invention are distinguished by a high moisture absorbency. Water is reversibly bonded by absorption from both the liquid and the vapor phases. The moisture absorbency of the molding compounds ranges between 3 and 10 wt. %, and for the preferred molding compounds between 4 and 8 wt. %. Consequently, the hydrophilic molding compounds of the invention possess, compared with conventional polyesters, a surprisingly high moisture absorbency. The following table lists some figures for a conventional polyethylene terephthalate fiber and for a polyethylene terephthalate fiber, obtained according to the invention, which contains 7.5 wt. % of a lithium/calcium polyacrylate (mole ratio Li:Ca=60:40) cross linked by means of 2 mole % divinyl benzene. Determinations were performed in accordance with DIN 54 201 at a constant atmosphere of 20.degree. C./65% RH (standard climate according to DIN 50 014), 34.degree. C./92% RH and 40.degree. C./92% RH (warm-humid constant climate as per DIN 50 015):
______________________________________ Moisture Uptake (in wt. %) For Polyethylene Terephthalate Fiber Fiber According To Climate Conventional The Invention ______________________________________ 20.degree. C./65% RH 0.3 1.8 34.degree. C./92% RH 0.5 5.6 40.degree. C./92% RH 0.6 6.3 ______________________________________
It is clear from the table that the polyester fibers of the invention exhibit in a standard atmosphere of 20.degree. C./65% RH, as well as at higher temperatures and higher moisture levels, a substantially higher moisture absorbency than conventional polyesters. In this context, and in terms of wearing comfort, the absolute moisture absorbency is of less importance than the difference in moisture uptake between that observed at 20.degree. C./65% RH and that observed during the actual wearing of a garment.
The constant condition of 34.degree. C./92% RH is known to correspond to body conditions at the skin at the upper limit of the comfort range, and the difference in moisture uptake, as a result, corresponds to the moisture uptake capability of the textile material when worn to the limit of the comfort range. For standard polyethylene terephthalate, the difference by weight is only 0.2%, whereas for the hydrophilic polyester fiber of the invention it is about 4.0%.
Another important property of the fiber of the invention is the high water retention. Water retention is usually determined according to DIN 53 814 wherein a specific quantity of the material to be investigated is completely soaked with water in combination with a wetting agent and then centrifuged under precisely defined conditions. The centrifuged specimen is weighed, dried and weighed once again. The difference between the two weighings represents the water retained after centrifuging of the specimen. Standard polyester has, as a rule, a water retention of about 2 to 5% by weight, whereas the fiber of the invention has a water retention of 8 to 20%, and preferably 10 to 15%. The water retention of a textile material is of decisive importance, in terms of the use characteristics of the material.
Another important characteristic of textiles, in terms of wearing comfort, is the moisture perception limit. It indicates at what moisture content (in % by weight) a textile specimen feels moist.
Two methods were employed to determine the moisture perception limit. In one method, use was made of dried textile specimens, e.g., flat knit samples, which were exposed to an increasingly more humid atmosphere. In the other method, use was made of textile specimens which were wetted, in keeping with the process for the determination of the water retention (DIN 53 814), and then dried in a standard atmosphere (20.degree. C. and 65% RH). In both cases, a panel of at least 4 persons was used to judge whether T-shirts constructed of the textile specimens feel dry enough to be comfortably worn under the test conditions. Using these methods, the moisture perception limit of standard polyester was found to be about 0.4% by weight and of cotton about 8% by weight. The moisture perception limit of the fibers of the invention was found to be about 5 to 6% by weight.
Also, in terms of moisture release, the fibers of the invention display excellent properties. The parameter of "moisture release" characterizes, in a relative way, the time required for a specimen which has been pre-moistened according to DIN 53 184 in a standard atmosphere of 20.degree. C. and 65% RH, to release a specified amount of moisture. Of particular significance in moisture release determinations is the time which must elapse before moisture perception limit is reached. If the relative time for this to occur for conventional polyethylene terephthalate is taken to be 1, then the relative times for the hydrophilic polyester of the invention, wool and cotton have been found to be 1.3, 2.5 and 3, respectively. In other words, it has been found that, with the hydrophilic polyesters of the invention, the point during drying at which the textile material feels dry is reached much faster than with wool or cotton.
In addition to the above-described properties relating to hydrophilic nature, fibers produced from the molding compounds of the invention also exhibit excellent textile properties comparable to those of conventional polyester fibers. The fibers of the invention can be spun in the conventional deniers and possess good textile properties.
The dye affinity of the fibers of the invention is very good, as well. In fact, the dye uptake of the polyester fibers of the invention is greater than that of conventional polyester fiber.
Optical properties of the fibers produced from the hydrophilic molding compound of the invention correspond essentially to those of TiO.sub.2 -pigmented polyester fibers, i.e., they are deep/dull. Therefore, the addition of TiO.sub.2 is not absolutely necessary for delustering.
The hydrophilic properties of the molding compounds of the invention are permanent. They are preserved during conventional further processing and during wear. Indeed, a significant increase in hydrophily is observed under the influence of home laundering. The invention is explained in further detail by way of the following examples: